This invention relates generally to Sagnac effect rotation sensors and particularly to fiber optic rotation sensors that guide counterpropagating light waves in a sensing loop to measure rotations about a sensing axis perpendicular to the plane of the sensing loop. Still more particularly, this invention relates to fiber optic rotation sensors that use [3.times.3] couplers to supply optical signals to the sensing loop and to guide the optical output signals from the sensing loop to electrical apparatus that processes the optical output signals to determine the rotation rate.
Fiber optic rotation sensors are well-known in the art. Previous fiber optic rotation sensors included evanescent field couplers to couple light between two lengths of optical fiber. Subsequently, fiber optic rotation sensors using [3.times.3] couplers were developed. The primary advantage of using a [3.times.3] coupler in a fiber optic rotation sensor is the ease with which such devices are interfaced with electronics.
U.S. Pat. Nos. 4,440,498 and 4,479,715 to Sheem disclose two fiber optic rotation sensors that include [3.times.3] couplers. U.S. Pat. No. 4,440,498 is directed to a fiber optic rotation sensor that includes a fiber optic sensing loop and an input fiber. A [3.times.3] fiber optic coupler divides light between the input fiber and the two legs of the fiber optic sensing loop.
U.S. Pat. No. 4,479,715 discloses a Sagnac effect rotation sensor in which the ends of a fiber optic sensing loop are coupled to a pair of optical waveguides. Light is input to a central input waveguide that is between the optical waveguides that are coupled to the ends of the sensing loop fiber. The three optical waveguides are arranged to form a [3.times.3] optical coupler. The input light is coupled from the central input waveguide to the optical waveguides that are connected to the optical fiber coil to produce the counterpropagating waves in the fiber optic sensing loop. The waves traverse the sensing coil and combine in the coupler. The combined waves are detected, and the resulting electrical signals are processed to determine the rotation rate.
U.S. Pat. No. 4,944,590 to Poisel et al. discloses an optical fiber gyroscope that uses a [3.times.3] coupler to couple optical signals into and out of a fiber optic sensing loop. Poisel et al. discloses a photodetector arranged to detect the light that has been input the [3.times.3] coupler that is not coupled into the fiber optic sensing loop. The electrical signal resulting from detecting this light is used in signal processing circuitry to make adjustments for variations in the input light intensity.
Such fiber optic rotation sensors may be operated in phase quadrature, which provides maximum sensitivity at zero rotation rate. Unfortunately, previous fiber optic rotation sensors that include [3.times.3] optical couplers are sensitive to temperature changes. The coupling ratios of the [3.times.3] fiber optic couplers are temperature-sensitive such that bias errors of 1000.degree. per hour are typically observed. Errors of such magnitude are unacceptable for most applications of rotation sensors.